1) Field of the Invention
The present invention relates to a method of producing a corrugated metal sheeting comprising a flat metal foil or sheet and a corrugated metal foil or sheet joined to each other, which sheeting is used to produce a carrier or honeycomb structure for carrying catalytic agents used for purifying exhaust gases from, for example, an internal combustion engine of an automobile. The invention also relates to a honeycomb structure produced from the corrugated metal sheeting.
2) Description of the Related Art
It is well known that the honeycomb structure for carrying the catalystic agents can be formed of a ceramic material, for example, based upon cordierite, exhibiting a superior heat resistance. Nevertheless, this ceramic honeycomb structure is inherently brittle and easily broken by an impact thereof. Accordingly, recently a metal honeycomb structure which is not brittle has been developed and is in practical use.
The metal honeycomb structure is produced by forming a roll of a long corrugated metal sheeting or by stacking a plurality of honeycomb-like panels into which the corrugated metal sheeting is cut. The corrugated metal sheeting is produced by joining a flat metal sheet or foil and a corrugated metal sheet or foil to each other by brazing or welding. These flat and corrugated metal sheets may be formed of stainless steels, high-alloy corrosion-resistant steel or the like, and may have a thickness of about from 45 .mu.m to about 1 mm. The metal honeycomb structure produced from the corrugated metal sheeting is received in a metal casing open at each end, so that the honeycomb end faces of the metal honeycomb structure are exposed from each of the openings of the metal casing. The metal casing may be also formed of stainless steels, corrosion-resistant high-alloy-steel or the like, and may have a thickness of about 1 to about 2 mm. The honeycomb structure is fixed in the metal casing by brazing or welding, and catalytic agents are then applied to the metal honeycomb structure in a well-known manner.
In use, the metal casing with the honeycomb structure carrying the catalystic agents is incorporated in an exhaust system of an internal combustion engine of an automobile, and the honeycomb structure is subjected to severe thermal stress because it may be exposed to wide and sudden changes of temperature, for example, from about -25.degree. to about 900.degree. C., and thus the flat and corrugated metal sheets must be securely joined to each other so that they will not be separated due to the severe thermal stress.
It is possible to securely join the flat and corrugated metal sheets, which can endure the severe thermal stress, by brazing. Nevertheless, the joining of the flat and corrugated metal sheets to each other by the brazing is cumbersome and complicated, and further the brazing per se is expensive.
Unexamined Japanese Patent Publication (KOKAI) No. 62-71547 discloses a method of producing the corrugated metal sheeting wherein the flat and corrugated metal sheets are joined to each other by spot-welding. In particular, the flat and corrugated metal sheets are intermittently fed to a nip gap between a roller-shaped electrode and a gear-shaped electrode in such a manner that the corrugated metal sheet is engaged with the gear-shaped electrode and the flat metal sheet is in contact with the roller-shaped electrode. During the passage of the flat and corrugated metal sheets between the roller-shaped and gear-shaped electrodes, which are electrically energized, apexes of the corrugations of the corrugated metal sheet are spot-welded to the flat metal sheet.
This method has an inherent drawback in that the production is not efficient, i.e., the rate of production is limited because of the intermittent feeding of the flat and corrugated metal sheets. Also, the corrugations of the corrugated metal sheet are often subjected to a resilient stress during the welding, because a location at which the corrugations of the corrugated metal sheet should be welded is restrained due to the engagement of the corrugations with the gear-shaped electrode. Accordingly, the corrugations welded to the flat metal sheet are often easily separated therefrom because of the residual resilient stress, and this is, of course, further facilitated by the severe thermal stresses.
The above-mentioned Japanese publication also discloses that the honeycomb structure is produced by forming a roll of the corrugated metal sheeting comprising the flat and corrugated metal sheets spot-welded to each other, and that the roll having a honeycomb structure is united by welding edges of the honeycomb end faces thereof to each other, so that the roll having a honeycomb structure will not be unrolled. Nevertheless, in use, the welded edges may be separated from each other due to the severe thermal stresses, because these thermal stresses tend to be concentrated at the honeycomb end faces of the honeycomb structure, and further, when the welded edges of the honeycomb end faces are separated from each other, a core portion of the honeycomb structure may be telescopically extended due to a flow pressure of the exhaust gas and vibrations generated during the running of the automobile.
Unexamined Japanese Patent Publication (KOKAI) No. 64-40180 discloses a method of producing a honeycomb structure wherein the flat and corrugated metal sheets are formed into a roll while being joined to each other by resistance-welding. In particular, the flat and corrugated metal sheets are tangentially fed to a winder at two diametrical locations thereof in counter feed directions, and first and second pairs of tip electrodes are transversely disposed with respect to the flat and corrugated metal sheets, and are in contact therewith in the vicinity of locations at which they are rolled in, respectively. Therefore, just before the flat metal sheet is rolled in it is welded to the rolled-in corrugated metal sheet by the first pair of tip electrodes in contact with the flat metal sheet, and just before the corrugated metal sheet is rolled in it is welded to the rolled-in flat metal sheet by the second pair of tip electrodes in contact with the corrugated metal sheet. Accordingly, in the thus-produced honeycomb structure, the rolled-in flat and corrugated metal sheets are welded to each other and thus cannot be unrolled.
Nevertheless, in this production method, each of the corrugations of the corrugated metal sheet is only welded to the flat metal sheet at one or two local portions, and accordingly, it is difficult to maintain the unity of the honeycomb structure under a severe thermal stress. Of course, for example, if the first and second pairs of tip electrodes are traversely moved step by step so that the resistance-welding can be repeatedly carried out along each of the corrugations of the corrugated metal sheet, it is possible to obtain a secure joining between the flat metal sheet and each of the corrugations, but in this case, the production rate is very low.